The present invention relates to a monitoring apparatus for monitoring a charge amount of a battery which is used for information processing equipment such as a personal computer of a notebook type or the like and, more particularly, to an apparatus for monitoring a charge amount of a battery in order to accurately notify the user of an operable time of equipment by the battery to the user.
In recent years, in association with the spread of portable or lap-top type information processing equipment, an apparatus which operates by using a battery as a power source is needed. In such information processing equipment, in order to assure a validity of data stored in a memory or disk, a normal finishing process of a program which is being executed, a data storing process, and similar procedures must be completed a predetermined time before the battery power is fully consumed and the power source is turned off. When the equipment operates by using the battery as a power source, therefore, it is necessary to notify the present residual capacity of the battery to the user. Since the capacity of the battery cannot be directly measured, however, it is necessary to detect charge and discharge currents of the battery and to convert to a charge amount and to monitor it. Such a charge amount of the battery is referred to as a capacity of the battery.
FIG. 1 shows a typical circuit construction of a conventional power capacity monitoring apparatus. For simplicity of explanation, FIG. 1 shows a circuit of a discharge current of a battery. A circuit construction of a charge current is similar to that of the discharge current except that the direction of the current is opposite.
In FIG. 1, in order to measure a discharge current flowing from a battery 10 to a load 12, a micro current detection resistor Rs is serially inserted into a discharge line. Although the current detection resistor Rs is inserted to the line on the positive electrode side of the battery 10, it can be also inserted to the line on the negative electrode side. A voltage V1 generated across the current detection resistor Rs is proportional to a discharge current Id which flows through the current detection resistor Rs. The detection voltage V1 is amplified by an operational amplifier 40, the amplified voltage is converted to a digital value by an A/D converter 42, and the digital value is read by a control MPU 22. Thus, a discharge current value at a certain moment can be known by the control MPU 22. Since it is a battery capacity that is actually measured, a current value (ampere) must be converted to a capacitance value (coulomb). In the conventional apparatus, the discharge current is measured every predetermined time by a program of the control MPU 22 and a time integration value (current x time) is obtained by a numerical value calculation, thereby obtaining the capacity value of the battery.
In such a conventional battery capacity monitoring apparatus, however, there are the following problems. First of all, the load current always fluctuates depending on an operating state of a circuit or a disk apparatus as a load. Therefore, an error due to a dynamic fluctuation of the load occurs in an interval time to measure the discharge current of the battery. The error increases as the interval time is long and the dynamic fluctuation of the load is large. In order to solve the problem of such an error, it can be considered to reduce the interval time of the current detection to a short enough time that the error does not cause a problem. In this case, however, an arithmetic operation error by the integration calculation is increased. That is, in the control MPU 22, although the current and time are multiplied by a binary arithmetic operation, an operation error occurs each time the operation is executed once. Since the number of arithmetic operations increases as the interval time is reduced, the arithmetic operation error also increases. Since a complicated arithmetic operation routine such as a floating-point arithmetic operation cannot be usually used due to a capacity limitation of a program memory or the like and the multiplication can be executed only by using integers, the selection of the interval time is accordingly limited and such a time cannot be minimized. Further, in the conventional apparatus, since the measurement data is inputted to the control MPU 22, the A/D converter 42 to convert the analog detection voltage to the digital value is certainly needed. Although an A/D converter of a wide bit width has to be used in order to reduce a quantization error of the A/D converter 42, it causes an increase in costs. On the other hand, although there is an MPU having an A/D converter therein or the like, a bit width of such an MPU is usually limited to 8 to 12 bits and there is a limitation in reduction of the quantization error.